Most hydraulic system have a filter device to filter contaminants and foreign particles from the oil. Some of such filtering devices have a dedicated pump whose sole function is to circulate oil from the tank through the filter and back to the tank. Heretofore, those pumps have been driven at a constant rotational speed so that the flow rate of the oil passing through the filter is fairly constant. One of the problems frequently encountered in cold weather operation is that the oil becomes quite viscous and causes a high pressure drop across the filter element until the oil warms up. The high pressure drop results in the contaminants already trapped in the filter being driven through the filter and carried into the clean oil in the tank.
Thus, it would be desirable to have a filter device for a hydraulic system wherein the pressure drop across the filter is minimized when the oil is cold while providing an optimum flow rate through the filter once the oil reaches a predetermined operating temperature.
The present invention is directed to overcoming one or more of the problems set forth above.